Manufacture of sodium sulfides



A. w. sADDlNG-roN MANUFACTURE OF SODIUM SULPHIDES Filed oct. 9, '1945, l

Patented Oct. 15, 1946 MANUFACTURE OF SODIUM SULFIDES Arthur W. Saddington, Syracuse, N. Y., assignor to The Solvay Process Company, New York, N. Y., a corporation of New York Application October 9, 1943, Serial No. 505,596

(Cl. ,2S- 134) Claims. l

This invention relates to the manufacture of sodium hydrosulfide (NaHS) or sodium sulfide (NazS) or mixtures of both.

It is known that sodium hydroxide may be reacted with HzS to form NazS which may be treated with more I-IzS to convert Nass to NaHS, and it has been proposed to make sodium hydrosulflde by gassing an NazS-NaHS liquor with pure hydrogen sulfide.

In processes of the type to which the invention relates, the source of sodium is commercial caustic soda which contains appreciable quantities of impurities, such as iron, copper, nickel, manganese and silicon. These impurities do not discolor caustic soda to any great extent, and hence their presence in commercial caustic is unobjectionable. However, during HzS gassing of commercial caustic liquor, such impurities pass thru the process and are carried into the resulting NaI-IS liquor. Such impurities may be present in the NaHS liquor as soluble salts or suspended in finely divided condition. While probably largely sulfides, exact compositions of the impurities are not known, and whatever their nature, these substances are referred to herein as metallic impurities. The presence of even small amounts of these impurities in NazS or NaHS solutions results in products of very poor appearance because of the highly colored nature of the impurities after having been'subjected to H2S gassing. Accordingly, a major problem presented in manufacture of relatively pure sodium hydrosulde has been the elimina tion of such impurities.

Commercially available hydrogen sulde gases contain appreciable amounts of carbon dioxide as an impurity. The discoveries constituting this invention arose out of attempts to manufacture, in accordance with known procedure, relatively pure sodium hydrosulde from commercial caustic soda and hydrogen sulfide gases containing carbon dioxide. The prior art indicates that when an NazS-NaHS liquor is gassed with pure I-I2S sufiicient to convert all of the NazS content of the liquor to NaHS and gassing is continued utilizing a substantial excess of I-IzS, the metallic impurities in the liquor coagulate and precipitate in a lterable form. I noted that, in no instance, are coagulation and precipitation of metallic impurities effected where the sodium hydrosulfide liquor has been produced from I-I2S gas containing CO2, even if all the NazSy has been converted to NaHS and a further large excess of H2S has been employed. This observationled to a further nding that when using commercial caustic soda as the source of sodium, coagulation and precipitation of metallic impurities are not obtained consistently even when pure Hes gas is used and even though the liquor has been gassed with a large excess of H25. Thus, while the prior art indications are that coagulation and precipitation of metallic impurities may be had by gassing the liquor to such an extent that the liqliquor with respect to carbonate of sodium and NazS contents. While not identified with certainty, indications are that the carbonate of sodium present in the system is in the form'of NazCOa and for convenience will be referred to as such in this specification.

It will be understood that good grades of commercial caustic soda contain appreciable amounts, sometimes as much as O16-0.20%, of NazCOa. Further, synthetic I-IzS gas also contains appreciable quantity of CO2 as an impurity, while the more common hydrogen sulde gases, such as those formed in oil refining operations, often contain as much as 10-25% CO2 by volume.

A principal object of this invention lies in pro- Vision of a process for making sodium hydrosulde or sodium sulfide, of purity acceptable to the trade, from commercial raw materials which constitute a source of metallic impurities and sodium carbonate in the NaHS liquor resulting from HzS gassing. The invention also aims to provide a process which permits use of an HzS gas containing substantial amount of CO2. Further objects are provision of a process by which it is possible to avoid troublesome solid sodium sulfide phase during the NaHS producing reaction, to operate such reaction continuously and at relatively low temperature, and if desired to provide for quantitative utilization of hydrogen sulde.

The process of the invention comprises generally three major stages: first, formation of a suitable NazS-NaHS starting liquor utilizing commercial NaOH as a raw material; second, a reaction stage in which the NazS-NaI-IS liquor is contacted with hydrogen sulfide gas to convert NazS to NaHS; and third, a purification stage in which the NaHS liquor'discharged fromthe re` action stage is treated in accordance with certain principles of the invention to effect (a) reduction of the amount of available sodium carbonate present in the liquor, and (U) control of the composition of such liquor with respect to NazS con- 3 tent, whereby to bring about elimination from the system of metallic impurities brought into the process as impurities in the initial caustic soda.

The nature of the invention is such that a major advantage is that H28 gas containing a substantial amount of CO2 may be employed. Hence, a preferred embodiment, using H2S containing COz, is described below in connection with the accompanying drawing diagrammatically showing suitable apparatus.

In usual practice of the invention, the NazS-NaHS absorbing liquor in tank II is made by treating a portion of NaI-IS product liquor of a previous run with commercial sodium hydroxide which may contain say 0.l-0.2% or more by weight of NazCOs. However, to start the process an aqueous solution of sodium hydroxide of any suitable concentration, e. g. 50-'70% NaOI-I, may be gassed with hydrogen sulfide, preferably pure, to such an extent as to form a liquor containing NazS and a substantial amount of NaHS. In this preliminary step, the liquor mass should be kept at any suitable temperature, e. g. 70 C. or upwards depending upon the sodium sulde concentration of the liquor, needed to maintain all of the sodium suldes in the liquid phase. Preliminary starting liquor, as to composition and temperature, should be similar in all respects to the absorbing liquor formed during the usual practice of the process which is preferably continuous.

When the process is under way, a portion of the sodium hydrosulde liquor discharged from the I-IzS gassing zone is run continuously thru line I .into NazS-NaHS absorbing liquor make-up tank I I Iwhich may be steam jacketed to keep temperature high enough to hold all sodium sulfide in liquid phase. While HzS gassing zone efliuent liquor may contain a limited amount of Nats, such liquor is preferably an aqueous solution of sodium hydrosulfide containing substantially no NazS. A strong solution of NaOH from tank I2 is likewise continuously fed into make-up tank I I. Although an absorbing liquor containing any suitable relative proportions of NazS and NaHS may be employed, it is more desirable to regulate feed of NaI-IS liquor and sodium hydroxide solution to tank II in such a way as to maintain therein an absorbing liquor of certain composition with respect to Na2S and NaHS, purpose of this feature being to provide an absorbing liquor of such composition as to make possible ready maintenance of sodium sulfide in the liquid phase both in tank II and in the subsequent reaction stage in which NazS of the absorbing liquor is converted to NaHS, and further to facilitate carrying out the HzS contacting operation at relatively low temperatures. To afford accomplishment of these ends, feed of sodium hydrosulde liquor from line l and of NaOH solution from tank I2 to the make-up tank is regulated to provide therein an absorbing liquor desirably containing not more than by weight of NazS. Especially where it is desired to supply absorbing liquor to the HzS reaction zone at a relatively low temperature, e. g. 'l0-80 C., and at the same time avoid any solid phase formation either in the absorbing liquor make-up tank or in the reaction zone, it is preferred to control formation of the absorbing liquor in such a Way that the liquor contains not more than 10% by weight of NazS and at least 25% by weight of NaHS.

While the entire process may be practiced batchwise, one of the advantages afforded is continuous operation, and accordingly absorbing liquor is fed continuously from make-up tank H into the top of a tower I4 providing a reaction zone I5 in which the absorbing liquor is contacted preferably counter-current with I-IzS gas from an inlet pipe I6, which gas, in accordance with the invention, may permissibly contain substantial amounts` of carbon dioxide.

I have found that any carbon dioxide contained in the hydrogen sulfide gas reacts substantially quantitatively with NazS to form a reaction product which appears and is herein considered for purposes of illustration to be sodium carbonate. I further observe that most of this sodium carbonate immediately precipitates as a solid readily filterable form. Since the H28 gas used may contain substantial amount of carbon dioxide the quantity of solid sodium carbonate produced may be correspondingly substantial. When the process is carried out as a batch procedure, solid sodium carbonate readily settles to the bottom of the reaction vessel and causes no particular operating difficulty. However, to provide for continuous operation throughout, I nnd that because of formation of solid sodium carbonate it is not feasible to carry out the NaHS producing reaction in the more or less usual type of packed. liquor-gas contacting tower. An open spray tower, containing no packing and provided at the top with a spray head for introduction of NazS-NaI-IS absorbing liquor and at the bottom an inlet for the HzS gas, may be employed. Contact towers containing bailles 20 such as indicated on the drawing may be used, although in such instance baille arrangement should be such as to afford good contact of gas and liquor but at the same time permit downiiow of a relatively light slurry and discharge of all of the slurry from the bottom of the tower. The type of liquor-gas contacting tower suitable for use in practice of the continuous basis may be defined as an unobstructed reaction chamber, but it should be understood that the term unobstructed is used in the sense that while the reaction chamber rnay contain mixing baffles, construction of these baffles is such as not to cause plugging but permits retarded but steady downflow of liquor containing some solids in Isuspension.

Hence in the preferred continuous operation, absorbing liquor of the type described is run into the top of a contact tower of suitable design, a stream of I-IzS gas which may contain carbon dioxide is fed into the bottom, unused and inert gases leave the top of the tower thru pipe 2I, and reacted liquor is discharged from the bottom into pipe 22 provided with valve 23. Hydrogen 'sulfide and carbon dioxide react with NazS of the absorbing liquor to form NaHS and sodium carbonate, and any NazCOs impurity contained in the initial commercial NaOH passes thru the contacting Zone unchanged. As a whole, reactions eiiected in the contacting zone are exothermic and some heat is developed. Broadly, the absorbing operation may be carried out at any temperature high enough to keep all so-dium suliide's in the liquor phase under the prevailing conditions of operation. However, temperatures in the reaction zone above about C. are not desirable because of greatly accelerated corrosion of equipment. I find that by regulating the composition of the NazS-NaHS absorbing liquor as in the above indicated preferred conditions, it is possible to feed the absorbing liquor to the reaction zone at a low temperature, e. g. 'l0-80 C.,

phase but does not exceed the indicated maxi' mum temperature and does not require extraneous cooling.

While the carbon dioxide content of the I-IzS gas fed to the reaction Zone may vary considerably and may be usually about 10% by volume, ordinarily it is preferred thatl such gas contain a predominating amount of I-IzS and not more than about 25% CO2 by volume.

In practice of the emb-odiment being described, design of the gas-liquor contacting tower` and rates of feed thereto of absorbing liquor and HzS gas, whether the process is carried out batchwise, countercurrent, or co-current, are vregulated so that during the contacting operation reaction of the NazS content of the absorbing liquor-,fed to the contacting zone is effected to such an extent that the reaction zone eluent NaHS liquor contains not more than by Weight of NazS, and preferably substantially no NazS at all. `Reason for such conversions of the Naas content of NaI-IS will subsequently appear.

In usual operation, employing an I-I2S gas containing substantial amount of CO2, the NaI-IS liquor discharged from the bottom of tower i4 contains a substantial amount of suspended solid sodium carbonate which is readily separable and is removed by lter 25. The ltrate in pipe 26 contains in solution the metallic impurities such as iron, brought into the process as impurities in the commercial sodium hydroxide or picked up from the liquor make-up or gassing apparatus, and if any NaHS or NazS product Were recovered directly from the filtrate, such products would be badly discolored and contain these impurities in amounts in excess of trade specifications.

As above stated, prior literature indicates that when an NazS-NaHS liquor is gassed with enough H2S tc convert all NazS to NaHS, plus a substantial excess of HzS, the metallic impurities in the liquor coagulate and precipitate out in a lterable form. I have found, with respect to consistently securing coagulation and precipitation of metallic impurities, that the controlling factors are the amounts of carbonate of sodium and NazS present in the NaHS liquor, and that the use of an excess of II2S is not critical. For example, in one operation, in which a substantial excess of HzS was employed, and the resulting liquor contained 46% by weight of NaI-IS, no NazS, and carbonate of sodium in amount equivalent to 0.08% by weight of CO3 radical, the liquor remained dark brown and there was no coagulation and precipitation of metallic impurities.

I have found that if sodium hydrosulflde liquor is saturated with carbonate of sodium, or contains an amount of available (i. e. in solution or both in solution and in solid form) carbonate of sodium equivalent to more than 0.06% by Weight of CO3 radical, metallic impurities do not coagulate or precipitate even though the liquor contains no NazS and has been treated with a large excess of HzS, and regardless of Whether the liquor has been made by gassing With purel HzS or with I-IzS containing carbon dioxide. I nd that if available carbonate of sodium is substantially completely removed from an NaI-IS liquor, or removed to an extent such that the liquor does not contain an amount of available carbonate of sodium equivalent to more than 0.06% by weight of CO3 radical, metallic impurities coagulate and precipitate even if the liquor contains a substantial' amount of NazS. The preferred condition of operation is to bring about substantially complete removal `of available carbonate of sodium. The second factor critical as to effecting coagulation and precipitation of metallic impurities is the NazS content 0f the NaHS liquor. I have found that,`in conjunction With the maximum carbonate of sodium tolerance noted above, the NaHSA liquor should contain not more than 5% by Weight of NazS, and preferably substantially noNazS at all. The preferred conditions of substantial absence of available carbonate of sodium and of substantial absence of NazS, cause the most thorough and rapid coagulation and precipitation of metallic impurities. Other permissible conditions, as to available carbonate of sodium and Na2S contents, Within the limits stated bring about an ultimately satisfactory coagulation and precipitation of metallic impurities, the overall difference being that as the available carbonate of sodium and NazS contents increase to the maximum values noted, rates of coagulation and precipitation of metallic impurities correspondingly decrease. Hence, briefly, theinvention involves formation of sodium hydrosulde liquor containing Vcertain metallic impurities and also an amount of carbonate of sodium equivalent to more than 0.06% by Weight of CO3 radical, and comprises the steps of reducing the available carbonate of sodium content of the liquor to` an amount equivalent to not more than 0.06% by weight of CO3 radical, and controlling the composition of the resulting liquor so that such liquor contains not more than 5% by Weight of sodium sulfide.

In the more usual practice of the invention process, i. e. where the HzS gas contains substantial amount cf CO2 and the reaction zone liquor eflluent contains a comparable substantial amount of solid sodium carbonate, the clear liquor filtrate in line 26 contains soluble sodium carbonate to the point of saturation and may Well contain in solution say 0.2-0.5% or more by Weight sodium carbonate. In the modification being described,

valve 2l in pipe 28 is closed and valve 29 is open.

A portion of the filtrate runs into recycle line l0, and by suitable adjustment of valves 30 and 3l, a portion of the filtrate corresponding with the make of one cycle of the process is run thru valve 3| into a tank 32 in which the NaHS liquor is treated in any suitable way to bring the sodium carbonate content down to the conditions stated. Carbonate reduction or elimination may be effected by treating the filtrate with a compound of an alkaline earth metal, e. g. calcium compound capable of reacting with the soluble sodium carbonate to precipitate the combined CO3 as calcium carbonate. The amount of treating material used should be at least sufficient to reduce soluble sodium carbonate content of the NaHS liquor to an amount equivalent to not more than 0.06% by Weight of CO3 radical. Preferably, the quantity of treating agent employed is that which is theoretically required to react with all of the soluble sodium carbonate. The preferred treat ing materialis calcium hydrosulde, and in this instance sodium carbonate is converted in accordance with the equation Ca (HS) z-l-NazCOsCaCOa-i-ZNaHS calcium carbonate being precipitated and the sodium of the NazCOa being converted to NaHS, the desired product. The calcium hydrosulfide treatment does not bring about any increase in the Na2S content of the treated liquor. In this instance, the control of the treated liquor composition, with respect to NazS content, is had by regulating the II2S gassing in tower I'll so that the NaI-IS liquor fed to the treating tank contains not more than 5% by weight of Naas or any lesser amount that may be desired in that particular operation. The foregoing procedure creates conditions which cause coagulation and precipitation of the metallic impurities in. readily filterable form. The treated liquor is then filtered in lter 34 to remove the calcium carbonate and the metallic impurity precipitates.

In place of calcium hydrosuliide, other treating material such as Ca(OH)2 may be employed. In this instance Ca(OI-I) 2 reacts with NazCOa to form CaCOa and NaOH ywhich in turn reacts with NaHS to form NazS, thus increasing the NazS content of the NaI-IS liquor Iundergoing treatment. Should the quantity of Ca(OI-l) 2 used be suicient to raise the NazS content of the treated liquor to above the maximum permissible Naas value for that particular operation, after the Ca(OI-I)2 treatment, the liquor in tank 32 may be gassed with enough H28, preferably pure, to lower the Nags content to the desired amount. In this mode of operation, control of the NazS content of Ca(OH)2 treated liquid is had partly by gassing in tower I4 and partly by gassing in tank 32.

It will be noted that even when Ca(HS) 2 is used as a treating agent, it is not critically important that the NaHS liquor fed into tank 32 contain not more than the permissible Nazs maximum for that operation. For example, an Nazs liquor containing say 6% by weight of NazS may be fed into tank 32, treated with Ca(I-IS)2, and thereafter gassed with enough H25, preferably pure, to reduce the Na2S content to the point desired. Hence, it will be understood that control of the composition, with respect to Naz-S content, of the treated liquor may be elected either before or after, or partly before and partly after treatment with the sodium carbonate content reducing agent, and unless specically stated to be contrary, the appended claims are to be so construed.

In some modications the reaction zone eff fluent may contain only a relatively small amount of solid sodium carbonate, e. g. where the HzS gas contains no or only a small amount of CO2. In these instances, Valve 21 may be opened, valves 23 and 29 closed, lter 25 by-passed, and filtration of the reaction eiiluent may be omitted. However, in such circumstances the amount of treating agent used in tank 32 should be sufficient to react with all available sodium carbonate (i. e. both solid and soluble) present at the time of addition of the calcium compound to an extent such that the treated liquor does not contain more available carbonate of sodium than is equivalent to 0.06% by weight of CO3 radical. Otherwise, procedure is the same.

In practice of the invention, when treating NaHS liquors, containing as high as L1.2% by weight of NazS, in such a way as to reduce the available carbonate of sodium content substantially to zero, straw yellow product liquors have been obtained containing as little as 0.0002% FezOa and 0.045% SiO2 by weight.

While the invention has been described chie-fly in connection with use of an H28 gas containing CO2, it is noted that the invention is not thus restricted. It will be understood that many grades of commercial caustic contain sufficient Na2CO3 as an impurity to result in formation of an NaI-IS liquoi` containing more than the indicated tolera-- ble maximum of carbonate of sodi-um. The

principles of the invention apply to purication of NaHS liquors containng an amount of available carbonate of sodium equivalent to more than 0.06% by weight of CO3 radical, no matter how such liquors are produced.

It will be understood that if the final product desired is VNaHS, gassing in the reaction Zone, and in tank 32 if needed, is carried out to an extent surlcient to convert all the NazS to NaHS. On the other hand if an NazS product is desired, except as indicated, complete conversion of NazS to Nal-IS is unnecessary, since the liquor eilluent of lter 34 may be treated with NaOl-I to cut back the product to NazS. If desired, the NaI-IS or NazS may be converted to solid form by evaporation of the liquors and solidiiication by known methods.

I claim:

1. The method for producing puried sodium hydrosulde from a sodium hydrosuli'lde liquor containing (a) at least one metallic impurity of the group consisting of iron, copper, nickel, manganese and silicon, and (b) an amount of available carbonate of sodium equivalent to more than 0.06% by weight of CO3 radical, which method comprises treating said liquor with an alkaline earth compound so as to react with available carbonate of sodium to precipitate combined CO3 thereof as alkaline earth carbonate, the amount of said alkaline earth compound being at least sufficient to reduce the available carbonate of sodium content of said liquor to an amount equivalent to not more than 0.06% by weight of CO3 radical, controlling the composition of the resulting liquor so that such liquor contains not more than 5% by weight of sodium sulfide, thereby effecting coagulation and precipitation of metallic impurities, and separating said metallic impurities from such liquor.

2. The method for producing purified sodium hydrosuli'lde from a sodium hydrosulde liquor containing (a) metallic impurities of the type present in commercial sodium hydroxide, and (b) an amount of available carbonate of sodium equivalent to more than 0.06% by Weight of CO3 radical, which method comprises treating said liquor with an alkaline earth compound so as to react with available carbonate of sodium to precipitate combined CO3 thereof as alkaline earth carbonate, the amount of said alkaline earth compound being at least sufficient to reduce the available carbonate of sodium content of said liquor to an amount equivalent to not more than 0.06% by weight of CO3 radical, controlling the composition of the resulting liquor so that such liquor contains not more than 5% by weight of sodium suliide, thereby effecting coagulation and precipitation of metallic impurities, and separating said metallic impurities from such liquor.

3. The method of producing puried sodium hydrosulde which comprises forming an NazS- NaHS liquor containing metallic impurities of the type present in commercial sodium hydroxide, contacting Said liquor with an HzS gas under temperature conditions high enough to maintain sodium sulde in liquid phase, continuing contact of the liquor and HzS gas for an interval sufcient to elect conversion of at least a major portion of the NazS content of said liquor to Nal-IS thereby forming an NaI-IS liquor containing, as a result of presence of oxide of carbon impurities in the system, an amount of available carbonate of sodium equivalent to more than 0.06% by weight of CO3 radical, reducing the available carbonate of sodium content of said NaHS liquor to an amount equivalent to not more than v0.06% by Weight of CO3 radical, controlling the composition of the resulting NaHS liquor so that such liquor contains not more than 5% by weight of sodium sulfide, thereby effecting coagulation and precipitation of metallic impurities, and separating said metallic impurities from such liquor.

4. The method of producing purified sodium hydrosulde which comprises forming an NazS- NaI-IS liquor containing metallic impurities of the type present in commercial sodium hydroxide, contacting said liquor with an H25 gas containing CO2 as an impurity under temperature conditions high enough to maintain sodium sulfide in liquid phase, continuing contact of the liquor and HZS gas for an interval sufficient to effect conversion of at least a major portion of the NazS content of said liquor to NaHS thereby forming an Nal-IS liquor containing an amount of available carbonate of sodium equivalent to substantially more than 0.06% by weight of CO3 radical, reducing the available carbonate of sodium content of said NaHS liquor to an amount equivalent to not more than 0.06% by weight of CO3 radical, controlling the composition of the resulting Nal-IS liquor so that such liquor contains not more than 5% by Weight of sodium sulde, thereby effecting coagulation and precipitation of metallic impurities, and separating said metallic impurities from such liquor. y

5. The method of producing purified sodium hydrosulde which comprises forming an NazS- NaHS liquor containing metallic impurities of the type present in commercial sodium hydroxide, contacting said liquor with a gas containing predominantly H2S and a substantial amount of CO2 as an impurity under temperature conditions high enough to maintain sodium sulfide in liquid phase, continuing contact of liquor and HzS gas 'for an interval sufficient to effect conversion of NazS to NaHS to such an extent that the resulting NaI-IS liquor contains not more than 5% by Weight of NazS, said NaHS liquor containing carbonate of sodium in solution and as a solid precipitate, separating solids from said NaHS liquor thereby recovering NaI-IS liquorcontaining in solution an amount of carbonate of sodium equivalent to more than 0.06% by weight of CO3 radical, treating said recovered NaHS liquor with calcium hydrosulde in amount substantially equivalent to that needed to precipitate soluble combined C03 as calcium carbonate, thereby eiecting coagulation and precipitation of metallic impurities, and separating said metallic impurities from said recovered NaHS liquor.

6. The method of producing puried sodium hydrosuliide which comprises forming an NazS- NaHS liquor containing metallic impurities of the type present in commercial sodium hydroxide, continuously introducing said liquor into a relatively unobstructed reaction zone and contacting said liquor therein with a gas containing predominantly H and a substantial amount of CO2 as an impurity under temperature conditions high enough to maintain sodium sulfide in liquid phase, regulating contacting of liquor and HzS gas in said zone for an interval suilicient to effect conversion of Naas and NaHS to such an extent that the NaI-IS liquor formed contains not more than 5% by .weight of NazS, said NaHS `liquor containing carbonate of sodium in solution and as a solid precipitate, continuously withdrawing such NaI-IS liquor and suspended solids from the reaction zone, separating solids from said Nal-IS liquor thereby recovering NaHS liquor contain.1

ing in solution an amount of carbonate of sodium equivalent to more than 0.06% by weight of CO3 radical, treating recovered NaHS liquor with a calcium compound'so as to react With soluble carbonate of sodium to precipitate combined CO3 thereof as calcium carbonate, the amount of said calcium compound being at least sufficient to reduce the soluble carbonate of sodium content of said recovered NaHS liquor to an amount equivalent to not more than 0.06% by Weight of` CO3 radical, controlling the composition o f the. result-` ing NaHS liquor so that such liquor contains substantially no sodium sulfide, thereby eiecting coagulation and precipitation of metallic impurities, and separating said metallic impurities from the NaHS liquor.

7. The method of producing puriiied sodium hydrosuliide which comprises forming an NazS- NaHS liquor containing metallic impurities of the type present in commercial sodium hydroxide, continuously introducing said liquor into a relatively unobstructed reaction zone and countercurrently contacting said liquor therein with a gas containing predominantly H2S and a substantial amount but not more than 25% by volume of CO2 as an impurity under temperature conditions high enough to maintain sodium sulde in liquid phase, regulating contacting of liquor and HzS gas in said zone for an interval sufcient to effect conversion of NaeS and NaI-IS to such an extent that the resulting NaHS liquor contains substantially no NazS, said NaHS liquor containing carbonate of sodium in solution and as a solid precipitate, continuously withdrawing 'such NaI-IS liquor andsuspended solid from the reaction zone, separating solids from said NaI-IS liquor thereby recovering NaHS liquor containing in solution an amount of carbonate of sodium equivalent to more than 0.06% by Weight of CO3 radical, treating recovered NaHS liquor with calcium hydrosuliide in amount substantially equivalent to that needed to precipitate soluble combined CO3 as calcium carbonate, thereby effecting coagulation and precipitation of metallic impurities, and separating said metallic impurities from said recovered NaI-IS liquor.

8. The method of producing puriiied sodium hydrosulde from a sodium hydrosuliide liquor containing (a) atleast one metallic impurity of the group consisting of iron, copper, nickel, manganese and silicon, and (b) an amount of available carbonate of sodium equivalent to more than 0.06% by weight of CO3 radical, which method comprises reducing the available carbonate of sodium content of said liquor to an amount equivalent to not more than 0.06% by weight of CO3 radical, controlling the composition of the resulting liquor so that such liquor contains not that needed to react with available carbonate of sodium to precipitate combined CO3 thereof as calcium carbonate, controlling the compositlon of the resulting liquor so that such liquor contains not more than 5% by weight of sodium sulde, thereby effecting coagulation and precipitation of metallic impurities, and separating said metallic impurities from such liquor.

l0. The method of producing purified sodium hydrosulde `from a sodium hydrosulde liquor containing (a) metallic impurities of the type present in commercial sodium hydroxide, and (b) an amount of available carbonate of sodium equivalent to more than 0.06% by weight of CO3 radical, which method comprises reducing the available carbonate of sodium content of said sodium hydrosulde liquor to an amount equivalent to not more than 0.06% by Weight of CO3 radical, controlling the composition of the resulting liquor so that such liquor contains not more than 5% by Weight of sodium sulde, thereby effecting coagulation and precipitation of metallic impurities, and separating said metallic impurities from such liquor.

ARTHUR W. SADDINGTON. 

